1. Field of the Invention
The present invention relates to a fixing device which performs fixing by passing paper carrying an unfixed image on a fixing rotary body in contact therewith, and more specifically, to a fixing device with a function to prevent overheat a fixing rotary body when paper is wound around the fixing rotary body.
2. Description of the Related Art
Generally, in image forming apparatuses, such as printers, copying machines, facsimiles or compound machines, in which an image is formed on paper with (dry) toner by using an image forming method, such as an electro-photographic method, an electrostatic recording method, etc., fixing devices are used for fixing an unfixed toner image (unfixed images) transferred onto the paper.
As such fixing devices, for example, a fixing device comprising a fixing roller which is heated to a predetermined temperature by a heater and rotated, and a pressure roller which is pressure-contacted with the fixing roller to form a fixing nip and rotated is known. Paper having an unfixed toner image transferred thereto is introduced into and passes through the fixing nip that is a pressure-contact region between the fixing roller and the pressure roller, and the toner in the unfixed image is heated and melted and simultaneously an image receiving surface of the paper is pressurized, so that a toner image is fixed on the paper.
Further, this type of fixing device is usually configured such that a temperature sensor detects the temperature of a circumferential surface of the fixing roller, and the control which turns on or off the heating operation of a heater is performed according to the temperature detected, thereby heating the fixing roller to a predetermined temperature. Moreover, if a temperature detected by the temperature sensor or a separate temperature sensor is an abnormal high temperature, the heating operation is stopped.
Meanwhile, in such a fixing device, a situation sometime occurs that the paper which has passed through the fixing nip is not peeled from the circumferential surface of the fixing roller, but is wound around as it is. This may result in an overheated state of the fixing roller due to excessive heating thereof by the heater.
In particular, when the temperature sensor is disposed in contact with the circumferential surface of the fixing roller (for example, a region in the vicinity of a central portion in the axial direction) inside its paper passage region, the temperature sensor detects the temperature of the fixing roller via the wound paper. Therefore, the temperature of the fixing roller cannot be accurately detected. In other words, it is often that an actual temperature of the fixing roller around which paper is wound is erroneously detected as a temperature lower than itself. As a result, the above-mentioned control which stops the heating operation of the heater cannot be appropriately performed. For this reason, the fixing roller becomes overheated, which results in damage to a resin layer, etc formed on its surface. Moreover, problems occur including causing deterioration of the quality of a fixed image, etc.
With respect to the above problem, for the purpose of preventing occurrence of the problem caused by winding of paper around the fixing roller, a fixing device is conventionally suggested in which a temperature detecting unit is disposed to contact a region of the fixing roller outside its paper passage region, and when the temperature detected by the temperature detecting unit exceeds a predetermined overheat temperature, an overheat preventing unit is provided for intercepting the power supply to a heater (See JP-A-11-242405).
However, the fixing device related to the above suggestion also has the following problems.
First, since the temperature detecting unit is disposed outside the widest paper passage region of the fixing roller, in order to accurately detect an abnormal overheat temperature in case paper with the largest width is wound around the fixing roller, it is necessary to further extend and dispose a heat generating portion of the heater by such an extra amount that a region heated by the heater extends to a region outside the widest paper passage region. It may deteriorate the density of thermal energy, which makes it impossible to efficiently heat the fixing roller by using the thermal energy.
Further, in the fixing device in which the fixing roller is heated by a plurality of independently controllable heaters, the temperature detecting unit should be respectively installed in conformity with the heating regions of the respective heaters. Therefore, the number of temperature detecting unit to be installed may increase with an increase in the number of heaters to be installed, which may increase the cost.